1. Field of the Invention
The present invention relates to a light conductive plate for a plane illuminating apparatus used as a back light in liquid crystal display devices such as a personal computer, a word processor, a telephone and the like. The present invention further relates to a liquid crystal image display device using the illuminating apparatus.
2. Description of the Related Art
In a side light source type plane illuminating apparatus, which is commonly called edge lighting, a transparent light conductive plate has a light reflecting surface on the back thereof, and a dimming pattern intended to evenly diffuse the light emitted from the light emitting surface is applied to either surface of the light conductive plate. This prevents uneven luminance, in which the portion of the emitting surface near the light source is brighter, and the portion of the emitting surface far away from the light source is darker.
A typical dimming pattern is shown in FIG. 6 (dot gradation printing). The dimming pattern is there composed of a plurality of circular disturbing reflecting surfaces of different sizes on the back of the transparent conductive plate. Those farther away from the light source L are larger so that the area of the reflected light increases with distance from the light source. In other proposals, a concave portion or a convex portion may be provided on the back of the light conductive plate, and the depth or height thereof may be successively increased (Japanese Patent Laid-Open No. 5-224019 and Japanese Utility Model Laid-open No. 5-697829).
The design wherein a concave portion or a convex portion is provided on the surface of the light conductive plate, and the density, size or like characteristic thereof is gradually changed to control a degree of reflection and dimming with respect to an incoming light so that it is uniformly emitted, is difficult to design and produce. In the system making use of control of incident light by a dot gradation printing (FIG. 6), the design and production are relatively easy but this system has a drawback in that the producing process comprises two steps, i.e. molding and printing. In addition, a sufficient luminance is not obtained.